1. Field of the Invention
The present invention relates to the art of earth boring. In particular, the invention relates to packers, bridgeplugs and corresponding methods and apparatus for selectively obstructing and reopening a well flow channel.
2. Description of Related Art
Well pipe such as coiled or threaded production tubing, for example, is surrounded by an annular space between the exterior wall of the tubing and the interior wall of the well casing or borehole wall. Frequently, it is necessary to seal this annular space between upper and lower portions of the well depth. Appliances for accomplishing the sealing function are known in the well drilling arts as xe2x80x9cpackersxe2x80x9d. Traditionally, the sealing element of a packer is a ring of rubber or other elastomer that is in some manner secured and sealed to the interior well surface which may be the interior casing wall or the raw borehole wall. By compression, for example, the ring of rubber is expanded radially against the casing or borehole wall.
xe2x80x9cBridgeplugsxe2x80x9d are well appliances for obstructing the flow continuity of an entire bore; whether it is the entire cross-section of the wellbore, the cross-section of a well casing or the cross-section of a production tube.
One of the greater utilities for a well packer or bridgeplug is to isolate a designated section of well bore along the wellbore length that penetrates a particular zone or earth strata. In some cases, the isolated zone may be burdened with an inordinately high internal pressure. For that reason, the packer or bridgeplug may be called upon to confine an unusually high pressure differential.
In other cases, where the packer engages the raw borehole wall to seal the annulus, for example, the packer must tightly and continuously engage a rough and highly irregular wall surface.
Either of the two examples above necessitate unusually high applications of setting force against the sealant to attain the degree rigidity and seal quality required with elastomers having the essential stiffness and other properties necessary to confine high differential pressure loads or expand into deep contours. However, high force and stress loads on a well tube also introduces the potential for other forms of tool and equipment failure.
It is an object of the present invention, therefore, to provide a well packer or bridgeplug having selectively controlled stiffness and other elastomer properties.
Also an object of the present invention is a packer or bridgeplug that may be engaged with relatively light force and when sealed, have the sealing element stiffened and rigidified.
Another object of the present invention is a well packer or bridgeplug having electromagnetically controlled stiffness properties.
Another object of the invention is a well packer or bridgeplug that is set with low force and stiffness properties which are thereafter switched or transformed to high stiffness properties and which may thereafter be switched or transformed back to the low stiffness property for retrieval of the packer if desired.
These and other objects of the invention as will be apparent from the following description of the preferred embodiments are attained by packers and bridgeplugs having a magnetorheological elastomer for the annulus or bore sealing element. Although the invention will be predominantly described in terms of a packer, it should be understood that the principles described are equally applicable to a bridgeplug.
xe2x80x9cControllable fluidsxe2x80x9d are materials that respond to an applied electric or magnetic field with a change in their rheological behavior. Typically, this change is manifested when the fluids are sheared by the development of a yield stress that is more or less proportional to the magnitude of an applied magnetic field. These materials are commonly referred to as electrorheological (ER) or magnetorheological (MR) fluids. Interest in controllable fluids derives from their ability to provide simple, quiet, rapid-response interfaces between electronic controls and mechanical systems. MR fluids are non-colloidal suspensions of polarizable particles having a size on the order of a few microns. Typical carrier fluids for magnetically responsive particles include hydrocarbon oil, silicon oil and water. The particulates in the carrier fluid may represent 25-45% of the total mixture volume. Such fluids respond to an applied magnetic field with a change in rheological behavior. Polarization induced in the suspended particles by application of an external field causes the particles to form columnar structures parallel to the applied field. These chain-like structures restrict the motion of the fluid, thereby increasing the viscous characteristics of the suspension.
Magnetorheological elastomers are magnetic field responsive elastomers that may be considered to be solid analogs of magnetic field responsive fluids. Like many field responsive fluids, field responsive elastomers are composed of polarizable particles dispersed in a polymer medium. The physical phenomena responsible for the field sensitivity of the elastomers is very similar to that of field responsive fluids. There are, however, some distinct differences in the way in which these two classes of materials are typically intended to operate. The most noteworthy is that the particle chains within the elastomer composite are intended to always operate in the pre-yield regime while field responsive fluids typically operate within a post-yield continuous shear or flow regime. Indeed, the strength of field responsive fluids is characterized by their field dependent yield stress while the strength of field responsive elastomers is typically characterized by their field dependent modulus.
Typically, during the manufacturing process for a magnetorheologncal elastomer, magnetic fields are applied to a polymer composite during crosslinking such that particle chain (columnar) structures form and become locked in place upon final cure. The formation of columnar particle structures within the elastomer composition corresponds to a low dipolar energy state. Flexure of the cured composite in the presence of the field causes particle displacement from this low energy state, thereby requiring additional work. In principle, this required additional work rises monotonically with applied field, thus resulting in a field dependent shear modulus.
Magnetorheological foams are devices that contain MR fluid that is constrained by capillary action in an absorbent matrix such as a sponge, open-celled foam, felt or fabric. The absorbent matrix serves to keep the MR fluid located in the active region of the device between the poles where the magnetic field is applied. The absorbent matrix requires only a minimum volume of MR fluid in the matrix to develop yield strength and resist shear motion. This basic arrangement may be applied in both linear and rotary devices wherever a direct shear mode would normally be used.
Because of their open structure, the shape of an MR fluid foam device is much less constrained than that of a normal controllable MR fluid device. Multiple degrees of freedom are easily accommodated.
Pursuant to the invention, packer seal elements are fabricated with magnetorheological elastomers or foams for disposition about an electromagnetic field winding embedded within and around a packer or bridgeplug mandrel. The winding may be connected by conductive cable to a surface power source. Alternatively, the winding may be powered by a circulating mud driven generator, for example.
For positioning downhole, the mandrel winding is de-energized. When positioned, the mandrel winding remains de-energized when the elastomer sealing elements are expanded to sealing engagement with the well bore or casing walls. After sealing, the mandrel windings are energized to stiffen the elastomer elements in the position and shape the elements were given while de-energized.